Our studies of cat motor nerve terminals have disclosed sharp pharmacologic differences between tonic and phasic terminals. These differences appear to relate to pre-junctionally located membrane receptors having varied specificities. Our preliminary experiments indicate nicotine-like receptors and possibly other types (adrenergic and glucocorticoid). We intend pharmacologically to define receptors of tonic and phasic motor nerve terminals and to relate receptor differences to differences in: (a) physiologic function; (b) drug action; (c) patho-physiology; and (d) trophic function. Just as tonic and phasic motoneurons are known to have important functional differences and in turn to influence trophically muscle contractile properties, their terminals appear to be important points of both functional and trophic differences. These terminals therefore should be sites at which a more selective pharmacologic control of transmission and trophism can be attained. Our investigations, with respect to both tonic and phasic motor nerve terminals, will be aimed at: (a) the possible negative and positive feedback roles of nicotinic receptors in transmission; (b) motor nerve terminals, via negative feedback, as the critical site of transmission block by depolarizing drugs (e.g., Sch); (c) identification of a facilitatory receptor; (d) glucocorticoid receptors and their possible functional, trophic and pathophysiologic roles; (e) pre-junctional receptor significance in experimental allergic myasthenia gravis; (f) pre-junctional receptors as triggers of a putative cAMP system in tonic motore nerve terminals. The proposed research has basic clinical and heuristic significance.